KR20050106386A - A method for preparing carbon nanotubes using alumina templates and carbon nanotubes - Google Patents

Abstract

The present invention relates to a carbon nanotube and a method for manufacturing the same, and to prepare a nanoparticle for catalyst having a uniform size and shape by evaporating after filling the metal ion solution in the pores of the porous alumina nano-frame having a uniform diameter and depth, The present invention relates to a method of controlling the diameter of carbon nanotubes using the nanoparticles.

Description

A method for preparing carbon nanotubes using alumina templates and carbon nanotubes}

Carbon Nanotube (CNT) is a kind of carbon allotrope made of carbon. It is a material in which one carbon is combined with other carbon atoms in the form of hexagonal honeycomb pattern to form a tube. This nanometer (nm = one billionth of a meter) is a very small area of material. Such carbon nanotubes have excellent mechanical properties, electrical selectivity, excellent field emission characteristics, high-efficiency hydrogen storage medium characteristics, and are currently being actively studied. Examples of the carbon nanotube synthesis include an electric discharge method, a thermal decomposition method, a laser deposition method, a plasma chemical vapor deposition method, a thermochemical vapor deposition method, an electrolysis method, and a flame synthesis method.

Carbon nanotubes alone exhibit conductor properties, but when stacked in multiple bundles, they exhibit semiconductor-like properties. In addition, the size is very small and the physical properties can be used as a memory device, an electronic device, etc. provides infinite applications.

As a method of synthesizing carbon nanotubes, an electric arc discharge method was mainly used, but various other methods are currently being studied. Representative synthesis methods are electric arc discharge, laser deposition method, pyrolysis deposition method, thermochemical vapor deposition method, and plasma chemistry. Vapor deposition. In the deposition method, a hydrocarbon gas such as acetylene, ethylene, methane, or benzene is mainly used as the source gas of carbon, and transition metals such as Ni, Co, Fe, or alloys thereof are used as the catalyst metal.

In particular, in the case of using a liquid catalyst metal solution as a catalyst for synthesizing carbon nanotubes, the liquid catalyst is applied onto a substrate using an inkjet method, a spray method, a dipping method, or the like, and dried to a desired shape on the substrate. Catalyst can be formed.

Carbon nanotube synthesis technology can be used to grow nanotubes vertically on a substrate, to pattern carbon nanotubes on a substrate by patterning catalytic metal on the substrate, or to use nanoscale electronic devices. There have been attempts to grow carbon nanotubes in a horizontal direction.

Among the methods for producing carbon nanotubes, a method for controlling tube growth is chemical vapor deposition. In this method, the diameter of the carbon nanotubes can be controlled by controlling the diameter of the catalyst nanoparticles.

Until now, metal nanoparticles prepared by vacuum deposition of catalytic metal on the surface of a substrate in a thin film form and then heated or etched were used as catalysts, or catalyst catalysts were precipitated in nano-frames having uniform diameter pores. The method of using, etc. is used. In this method, there is a disadvantage that the diameter of the catalyst particles is not uniform or the diameter cannot be freely controlled.

As such, until now, catalyst particles were prepared separately and used only for carbon nanotube synthesis. As a result, there is no known technique for preparing catalytic nanoparticles having a uniform diameter from the solution by filling a catalyst metal ion solution in a predetermined space such as pores of a nanoframe.

In this regard, the present inventors have studied a method for producing a nanotube having a uniform diameter. In addition, the method of manufacturing a nanotube that can freely control the diameter of the nanotube was studied.

In the present invention, a porous alumina nano-frame in which pores of uniform diameter are regularly arranged by a well-known 2-step anodization method is prepared and evaporated by filling a metal ion solution for catalyst in the pores of the nano-frame. Nanoparticles for catalysts of uniform size were prepared by the method. At this time, the present inventors have found that the catalyst size can be adjusted by controlling the concentration of the metal ion solution for catalyst or the depth or diameter of the alumina nano-pores, and thus the diameter of the carbon nanotubes can be adjusted.

Accordingly, an object of the present invention is to prepare a catalyst nanoparticles having a uniform diameter by filling the metal ion solution for the catalyst in the pores of the porous alumina nanoframe and evaporated, and the concentration of the metal ion solution or the pores of the alumina nanoframe It is to provide a method of controlling the diameter of the carbon nanotubes by controlling the size of the catalyst particles by changing the depth or diameter.

It is also an object of the present invention to provide a carbon nanotube with a uniform diameter produced using the above method.

The present invention is to prepare alumina nano-frame having pores on the surface; Depositing a metal in the pores of the alumina nano-frame; Filling a metal ion solution for a catalyst in the pores of the alumina nano-frame in which metal is precipitated, and then evaporating the solution; Expanding the pores of the alumina nanoframe after solution evaporation; And flowing hydrogen gas in a high temperature furnace to reduce the nanoparticles for the catalyst formed from the solution and pyrolysing the hydrocarbon gas.

In another aspect, the present invention is to prepare alumina nano-frame having pores on the surface; Depositing a metal in the pores of the alumina nano-frame; And evaporating the metal ion solution on the permanent magnet after filling the metal ion solution for the catalyst in the pores of the alumina nano-frame in which the metal is precipitated.

In preparing the alumina nano-frame, in order to obtain alumina nano-frame in which pores of uniform diameter are regularly arranged, preferably, the aluminum sheet is oxidized by a two-stage or multi-stage anodization method to prepare the alumina nano-frame. Can be. More preferably, at the last stage of the multistage anodization, the pores may be dug to lower the voltage to form branches at the bottom of the pores. By changing the voltage and the concentration of the electrolyte in the anodization method, it is possible to control the diameter, depth and spacing of the pores of the alumina nano-frame. As the diameter or depth of the alumina nanopore varies, the amount of the metal ion solution for the catalyst to be filled varies, and accordingly, the size of the catalyst metal nanoparticles generated by evaporation of the metal ion solution for the catalyst is changed, and thus the catalyst It is possible to control the diameter of the carbon nanotubes formed by.

In order to precipitate the metal in the pores of the alumina nano-frame, preferably, a metal ion solution may be injected into the pores and then reduced by an electrochemical method to produce a metal precipitate. At this time, the metal precipitates are deposited on the bottom of the pores of the alumina nano-frame, which is called a metal nanowire because it is precipitated in the form of a long, linear rod from the bottom of the pores having a diameter of nanometers.

When the metal is filled with a catalyst metal ion solution in the precipitated pores and then evaporated, the metal is preferably evaporated on the permanent magnet. When evaporated on the permanent magnet, the particles condensed in each pore are attracted to the magnet and condensed into a lump at the bottom of the pore, resulting in uniform particle size and size, resulting in the catalysis of the condensed nanoparticles. The diameter of the carbon nanotubes is uniform.

At this time, the metal particles of nanometer unit size condensed by the evaporation of the metal ion solution acts as a catalyst for carbon nanotube production according to the present invention. Since the catalyst is a nanometer-sized metal particle, it is also called a metal nanoparticle for a catalyst. The size of the metal nanoparticles for the condensation varies according to the concentration of the metal ion solution filled in the pores of the alumina nano-frame, and thus the diameter of the carbon nanotubes also changes. Therefore, the diameter of the carbon nanotubes can be adjusted by adjusting the concentration of the metal ion solution.

In the step of expanding the pores of the alumina nano-frame, preferably, the alumina nano-frame may be precipitated in a basic solution to allow the pores to expand. As a basic solution, for example, when alumina nanoframe is immersed in NaOH solution or KOH solution, the alumina constituting the nanoframe melts, and the particles attached to the outer surface of the pores of the nanoframe also fall off into the solution. That is, when the particles of various diameters having the catalytic activity formed on the surface of the nano-frame is removed, only nanoparticles of uniform size formed in the pores act as a catalyst, thereby producing carbon nanotubes of uniform diameter. The metal nanoparticles for the catalyst formed on the metal precipitates (metal nanowires) in the pores of the alumina nanoframes are relatively safe because they are in the pores and thus are unlikely to be lost in the base solution. In addition, since the pore diameter is expanded, gas can be smoothly introduced into the pores during carbon nanotube manufacturing, thereby promoting tube growth.

As the hydrocarbon gas is pyrolyzed in the alumina nano-frame in which the catalyst is formed, carbon nanotubes are formed and grow. As the hydrocarbon gas, acetylene, ethylene, methane, benzene and the like can be used.

Hereinafter, the present invention will be described in detail for each step.

(a) Preparation of Alumina Nano Frame

Typically, aluminum thin plates are oxidized to prepare alumina nano templates. In the fabrication of porous alumina nanostructures in which the pores are arranged regularly, a two-step or multi-step anodization method is used. The two-stage anodization method is an application of an anodization method. When manufacturing a porous alumina nanoplate using the anodic oxidation method, pores having a uniform diameter may be regularly arranged perpendicularly to the nanoframe [ H. Masuda and M. Satoh, Jpn. J. Appl. Phys., 35, (1996) L 126]. Anodizing is a kind of electrolysis method in which a direct current is flowed into an acid electrolyte solution such as oxalic acid, phosphoric acid, sulfuric acid, etc. by using a thin aluminum plate as a positive electrode and a carbon electrode as a negative electrode. In this case, aluminum is oxidized to become alumina layer. It is known that the pores are broken. In the two-stage anodization method, a thin aluminum plate is anodized for a long time (more than 10 hours) to form pores, and then the oxide layer (alumina layer) that forms the pores is immersed in an acid solution to remove all the same. It is a method of digging pores of desired depth. The multi-step anodization method is a method that is useful for producing nano-frames in which pores of uniform diameter are regularly arranged by repeating a process of digging pores, removing an oxide layer, and digging again. The spacing between the pores is proportional to the voltage applied when anodizing and the depth of the pores is proportional to the time of anodizing [J.W. Diggle, T.C. Downie, and C.W. Goulding, Chem. Rev. 69 (1969) 365. The pore diameter can be expanded in acids such as phosphoric acid or base solutions such as sodium hydroxide [D. Al Mawlawi, N. Coombs, and M. Moskovits, J. Appl. Phys. 70 (1991) 4421].

Preferably, the pore bottom is branched by a voltage drop method in the final step of the anodic oxidation after digging a predetermined depth of pores in an electrolyte such as sulfuric acid, oxalic acid and phosphoric acid. If necessary, the pore diameter can be extended in phosphoric acid or NaOH solutions.

By making the bottom of the pore into a branch, metal nanowires such as Co, which are elongated rod-shaped precipitates deposited on the bottom of the pore by an electrochemical method in the subsequent step, are prevented from escaping from the pore. Prevents metal nanowires from acting as catalysts in the production of carbon nanotubes. For reference, in order for a metal nanowire in a pore to act as a catalyst, the catalyst particles must be able to move, and the tube must be able to grow with a tip growth mechanism at the end of the growing tube. In other words, if metal particles do not move in pores, they cannot act as catalysts.

(b) precipitating a small amount of metal in the pores

Co ^{2 +,} Fe ^{2 +,} to precipitate a small amount of a metal by electrochemical method after immersing the nano framework the metal ion solution, such as Fe ^{3 +} in the pore. The metal ions are electron-reduced and filled with elongated rods from the bottom of the pores to form metal nanowires. The metal nanowire is an elongated metal rod filled from the bottom of the pore by reduction of metal ions. In general, one end of the rod is at the bottom of the pore and the other end is at the center of the pore. When carbon nanotubes are synthesized after the metal nanowires are deposited on the bottom of the pores (see FIGS. 2C and 3), much more carbon nanotubes grow than when the metal nanowires are not precipitated (see FIGS. 2A and 2B). Can be observed.

From these observations, catalyst nanoparticles formed as the metal ion solution evaporates can fall off the surface more easily when formed on the cross section of the metal nanowires deposited in the pores than when they are formed at the bottom of the alumina pores, resulting in more tube growth. Is considered.

(c) filling the pores with metal ion solution and evaporating

After filling the metal ion solution in the pores of the alumina nano-frame, the solution is evaporated. As the metal ion solution may be an ion solution of ^{Fe 2 +, Fe 3 + Co} 2 + or Ni ^{2 +.} Preferably, the metal ion solution may be evaporated on a permanent magnet. When carbon nanotubes are synthesized after evaporation of the metal ion solution on the permanent magnet, a tube having a uniform diameter grows (see FIGS. 2B and 3), but otherwise the diameter of the carbon nanotubes is not uniform (FIG. 2A, 2c).

Catalytic metal atoms and ions are paramagnetic and attracted to permanent magnets lying under pores. As the condensed particles precipitate as the solution evaporates, they are pulled to the bottom of the pore to condense into a single particle, and the tube grows due to the catalysis of the condensed particles, so that a tube of uniform diameter can grow. At this time, dozens or hundreds of atoms of the metal ion solution gather to form catalyst particles, and the diameter of the carbon nanotubes changes in proportion to the diameter of the particles.

Since the diameter and depth of the pores of the porous alumina nano-frame prepared by the anodizing method are very uniform, the concentration of the metal ion solution is controlled, so that the size of the metal catalyst particles precipitated while the solution evaporates can be controlled. (A), (b) and (c) of FIG. 3 show that Co is electrochemically so that Co nanowires have a length of about 400 nm in pores having a uniform diameter (35 nm) and depth (1 μm). After filling, carbon nanotubes grown from iron-catalyzed nanoparticles formed by filling solutions with iron concentrations of 1x10 ^-3 M, 3x10 ^-3 M, and 6x10 ^-3 M, respectively, with average diameters of 7 nm, 13 nm, and 17 nm, respectively. Very uniform.

By the above steps, the metal nanoparticles for the catalyst are prepared.

In addition, the carbon nanotubes using the catalyst metal nanoparticles are further subjected to the following steps.

(d) expanding the pores of the alumina nano-frame on which the catalytic metal is formed

The pores of the alumina nanoframe in which the catalytic metal is formed are expanded. There is a method of treating the alumina nanoframe with a base solution to expand the pores. That is, in the base solution such as NaOH or KOH, the alumina nano-frame in which the catalyst metal is formed is immersed for about 1 minute. Preferably, a solution of NaOH in the base can be used.

The expansion of the pores of the alumina nanoframe by immersion in the base solution serves two roles.

First, when the metal ion solution is evaporated, in addition to the catalyst metal nanoparticles formed in the pore-filled solution, the solution on the surface of the nanoframe evaporates to produce metal particles having catalytic activity on the surface of the nanoframe. As the alumina that forms the frame melts, the particles that adhered to the outer surface of the nanoframe fall off into the solution. That is, since the metal particles of different diameters formed on the outer surface of the nano-frame is removed, carbon nanotubes having a uniform diameter are manufactured from nanoparticles having a uniform diameter formed in the pores.

Second, because the pore diameter is expanded, the gas can be smoothly introduced into the pores during the production of carbon nanotubes, thereby promoting tube growth.

The pore expansion may be carried out before the particles precipitated in the pores are reduced to hydrogen gas or after the pore expansion.

(e) reducing the catalytic metal nanoparticles with hydrogen gas and pyrolyzing the hydrocarbon gas

Hydrogen gas is flowed into the alumina nano-frame to reduce the metal nanoparticles for the catalyst, and carbon nanotubes are prepared by pyrolyzing hydrocarbons such as acetylene and carbon monoxide.

The metal nanoparticles for the catalyst formed from the metal ion solution act as a catalyst for the pyrolysis reaction of the hydrocarbon to grow carbon nanotubes.

The present invention will be described in more detail with reference to the following Examples.

The following examples are intended to illustrate the present invention in detail, and the scope of the present invention is not limited by these examples.

Reference Example 1 Preparation of Alumina Nano Frame

After wiping the surface of aluminum plate (5cm × 2cm × 0.05cm), put it in 150ml of 0.3M oxalic acid solution, and use 40V DC at 17 ℃ with anode and carbon electrode as cathode. Spilled for 15 hours.

Subsequently, the alumina layer resulting from the first anodization was removed by immersing in a mixed solution of phosphoric acid (6 wt%) and chromic acid (1.8 wt%) at 60 ° C. for about 12 hours. After removing the alumina layer, the second anodization was performed for 15 minutes under the same conditions as the first anodization.

In addition, in order to make the pore bottom into a branch, in the final anodization, the voltage was lowered from 40 V to 10 V in four steps.

As a result, an alumina nano-frame having pores regularly formed on the surface was produced. (See Figure 5)

Diameter of the pores: 35 nm

Depth of pores: 1 ㎛

Spacing between pores: 104 nm, angle between pores 60 °

Reference Example 2 Preparation of Alumina Nano Frame

0.3 M sulfuric acid was used as an electrolyte instead of oxalic acid, and alumina nano-frames were prepared in the same manner as in Reference Example 1, with a voltage of 25 V dc and a temperature of an electrolyte solution of 0 ° C.

Diameter of the pores: 18 nm

Depth of pores: 1 ㎛

Spacing between pores: 65 nm, angle between pores 60 °

<Example 1>

After dipping the alumina nano-frame prepared in Reference Example 1 in a Co ^{2 +} solution of 0.4 M concentration, the AC power was applied using graphite as another electrode. Voltage was 15V is given a voltage is applied for 30 seconds, by reducing the Co ^{+ 2} ions in the solution to be precipitated from the bottom pores. The diameter of the precipitated Co nanowires was about 35 nm, the same as the pore diameter, and about 400 nm in length.

Subsequently, the pores were filled with an iron solution having a concentration of 1 × 10 ⁻³ M by the pore height, and left at room temperature for 3 hours on a permanent magnet of 100 Gauss to dry the solution to precipitate the metal nanoparticles.

The alumina nano-frame in which the metal nanoparticles were precipitated was immersed in a 0.1 M NaOH solution for 1 minute to expand the pores of the alumina nano-frame and remove external metal catalyst particles.

At this time, the metal catalyst particles inside the pores were treated very carefully in order to prevent them from coming out of the pores.

The expanded alumina nanoscale was placed in a tube furnace and contacted with hydrogen gas at a temperature of 600 ° C. and a flow rate of 100 ml / min for 1 hour.

Immediately, pyrolysis with acetylene gas to prepare carbon nanotubes. In this case, a mixture of acetylene (5%) and nitrogen was flowed at 100 ml / min for 10 minutes at a temperature of 600 ° C. to synthesize carbon nanotubes.

As a result, carbon nanotubes having a diameter of 7 nm and a length of 1 μm or more were obtained. (See Figure 3 (a))

<Example 2>

Carbon nanotubes were prepared in the same manner as in Example 1 except that an iron solution having a concentration of 3 × 10 ⁻³ M was used for the metal nanoparticle catalyst.

As a result, carbon nanotubes having an average diameter of 13 nm and a length of 1 μm or more were obtained.

<Example 3>

Carbon nanotubes were prepared in the same manner as in Example 1 except that an iron solution of 6 × 10 ⁻³ M concentration was used for the metal nanoparticle catalyst.

As a result, carbon nanotubes having an average diameter of 17 nm and a length of 1 μm or more were obtained.

Comparative Example 1

Carbon nanotubes were prepared in the same manner as in Example 1, except that the metal nanowires were not dried and dried in a place where there was no permanent magnet even when the metal ion solution was dried. The results are shown in Figure 2a. That is, as shown in Figure 2a, it can be seen that the diameter of the carbon nanotubes are not uniform, and the number of grown tubes is not too many.

Comparative Example 2

Carbon nanotubes were prepared in the same manner as in Example 1 without forming metal nanowires. The results are shown in Figure 2b. That is, as shown in Figure 2b, it can be seen that the diameter of the carbon nanotubes is uniform, but the number of the grown tube is not many.

Comparative Example 3

In the drying of the metal ion solution, carbon nanotubes were prepared in the same manner as in Example 1 except that the metal ion solution was dried in the absence of permanent magnets. The results are shown in Figure 2c. That is, as shown in Figure 2c, the carbon nanotubes grew in almost all pores, but it can be seen that the diameter of the carbon nanotubes are not uniform.

<Comparative Example 4>

Carbon nanotubes were prepared in the same manner as in Example 1, except that the pores of the alumina nano-frames were not expanded, that is, the alumina nano-frames on which the metal nanoparticle catalysts were formed were not treated in the basic solution. The results are shown in FIG. That is, as shown in Figure 4, in this case, it can be seen that there is a tube having a very large diameter of the carbon nanotubes.

When the nanocatalyst particles are produced by the method of the present invention, the size of the particles is very uniform. In addition, the size of the particles can be easily controlled by adjusting the concentration of the metal ion solution, and as a result, the diameter of the carbon nanotubes can be controlled.

1 is a schematic diagram showing a process of depositing a metal in a porous alumina nanoframe, filling a metal ion solution therein, forming a catalyst nanoparticle by evaporating the metal ion solution, and forming a carbon nanotube. .

Figure 2 is a case where the carbon nanotubes are prepared after filling the pores of the porous alumina nano-frame with a metal ion solution and evaporating the solution in the absence of permanent magnets and expanding the pores (a); (B) preparing carbon nanotubes by filling a metal ion solution in the pores of the nano-frame, evaporating them on the permanent magnet, and expanding the pores; And (c) preparing carbon nanotubes by depositing Co metal by electrochemical method in the pores of the nano-frame, filling the metal ion solution, evaporating the solution in the absence of permanent magnets, and expanding the pores. SEM (scanning electron microscope) photograph.

FIG. 3 is a SEM image of carbon nanotubes prepared after depositing a metal on the bottom of pores of an alumina nanoframe by electrochemical method, filling a metal ion solution for catalyst, evaporating the solution on a permanent magnet, and expanding pores. to be. (A), (b) and (c) in Figure 3 show the concentrations of iron ionic solutions 1x10 ^-3 M, 3x10 ^-3 M, and 6x10 ^-3 M, respectively, and the average diameters of the resulting carbon nanotubes are 7 nm and 13 nm, respectively. , 17 nm.

FIG. 4 is a SEM photograph of carbon nanotubes prepared without depositing pores of a nanoframe after filling a catalyst metal ion solution by electrochemical method at the bottom of the pores of an alumina nanoframe, drying with a catalyst metal ion solution. .

FIG. 5 is a SEM photograph of alumina nanoframes prepared by a two-step anodization using an oxalic acid solution.

Claims (9)

Preparing an alumina nano mold having pores on its surface; Depositing a metal in the pores of the alumina nano-frame; Filling a metal ion solution for a catalyst in the pores of the alumina nano-frame in which metal is precipitated, and then evaporating the solution; Expanding the pores of the alumina nanoframe in which the solution is evaporated; And Flowing hydrogen gas to reduce the nanoparticles and pyrolysing the hydrocarbon gas; Carbon nanotube manufacturing method comprising a. The method of claim 1, wherein the metal pore in which the metal is deposited is filled with a catalyst metal ion solution and then evaporated therein. According to claim 1, In the production of the porous alumina nano-frame, A method for producing carbon nanotubes, characterized in that to form pores of a predetermined depth arranged regularly by a multi-step anodization method. 4. The method of claim 3, wherein in the last step of the multi-stage anodization, the voltage is lowered to form branches at the bottom of the pores. The method of claim 1, wherein in the precipitating of the metal, the alumina nanoframe is immersed in an iron or cobalt metal ion solution to precipitate the metal at the bottom of the pores by an electrochemical method. The method of claim 1, wherein in the evaporation after filling the pores of the alumina nano-frame with a catalytic metal ion solution, the metal ion solution is iron or cobalt ion solution. The method of claim 1, wherein the expanding of the pores of the alumina nanoframe is to expand the pores by immersing the alumina nanotle in a base solution. 8. The method of claim 7, wherein the base solution is NaOH or KOH solution. Carbon nanotubes prepared by the method according to any one of claims 1 to 8.